Producing subsurface hydrocarbon deposits requires that a system be able to efficiently recover typically viscous and abrasive fluids from such deposits, typically 1,000 feet or greater below the surface, via a relatively small diameter casing (e.g., 2½″ to 9.0″ diameter). The challenges presented by such requirements has resulted in the development of a number of recovery systems. Of such systems, electric submersible pumping (ESP) has become one of the most widely applied systems for most field applications due to its high volume producing capability. The ESP system consists of a multi-stage downhole centrifugal pump directly driven by a downhole electric motor. Although proven to be effective, operating and servicing the system's downhole electric motor can be complicated and cost prohibitive. Moreover, use of an electrical pump downhole, such electrical pump and electrical wires leading thereto are potentially a source of electrical sparking. In a sometimes explosive environment of hydrocarbon and air as typically occurs downhole in a well, this can be accordingly extremely dangerous and thus highly undesirable, resulting in inability in some downhole applications to employ an ESP pump.
Addressing some of the shortcomings of the ESP systems are progressive cavity pumping (PCP) systems. A PCP system consists of a downhole progressive cavity pump actuated by a rod string that is rotated by a surface drive, typically an electric three phase motor, that can be easily operated and accessed for servicing, which progressive cavity pumps are further well suited for producing hydrocarbons from downhole explosive environments. Moreover, such downhole progressive cavity pumps, due to their “auger” design, are particularly suited to pump viscous, abrasive fluids found in “heavy oil” subsurface deposits. The performance of PCP systems, however, are known to be limited by speed and depth tolerance.
For efficient production, progressive cavity pumps typically require an operating speed of up to 1,200 rpm for best operation of the progressive cavity pump to maintain sufficient “head” to produce from depths at which viscous oil is typically found in North America, and in particular Alberta, in the Lloydminster, Alberta region.
Surface-driven rotation of the rod string does have vexing problems. Specifically, when a surface-driven PCP system is in operation, a significant quantity of energy is stored in the torsional strain of the rod string. The stored energy is released with backspin of the pump and/or rod string whenever the PCP system is shut down through routine operator intervention or automatic shut down, for example. When the power supply to the drive is lost or interrupted, the potential energy that remains in the system will cause the surface equipment and drive string to accelerate in the direction opposite its normal operating mode. Uncontrolled backspin can lead to surface equipment damage and backed-off rod strings or tubing. These conditions also pose a significant hazard to field personnel working on or near the surface equipment, and in some conditions, due to significant back-spin of a PCP drive unit at surface, have caused surface drive units to overspeed and fail, and in at least one instance fly apart and cause death to at least one individual located in proximity to the surface drive unit for a particular well. Thus, it is essential that braking mechanisms are provided to control the release of rod-string torque and restrict rod recoil to a safe speed. In many applications, if unrestrained by a braking mechanism, backspin speeds can increase to the point at which the drive fragments and radially explodes outward because of the high centrifugal forces generated.
U.S. Pat. No. 5,573,063 discloses a surface-driven pumping apparatus that can efficiently recover fluids from a deep well. In particular, such patent teaches a geared centrifugal pumping system (GCP) consisting of an uphole electric motor which uses flexible belts and pulleys 36, 40 at surface to reduce the driven speed of the rod string, which rod string downhole is connected to a downhole speed increasing gearbox and associated downhole pump assembly. The combination of the belts and pulleys at surface to reduce the driven speed of the rod string several-fold, and the speed-increasing transmission located downhole to increase the rotational speed of the pump several-fold in relation to the rotational speed of the rod string allows the rotational speed of the rod string to thereby be lowered within the operation limits of the system while at the same time maintaining high rotational speed of the driven downhole pump to thereby achieve the desired necessary pressure, output, and efficiency from such downhole pump. In particular, the drive unit comprising the uphole speed reducing belts and pulleys reduces the motor RPM (typically up to 1,200 rpm) to a speed at which the rod string can be stably rotated (about 500 rpm) thus reducing wear on the rod string (both frictional and fatigue loading due to back and forth bending of such rod string during rotation thereof), and the downhole geared transmission coupled at one end to the rod string and at another end to the downhole pump, is used to increase the speed of the rod string typically to the 1200 rpm range to thereby reach the optimum rpm for best operation of the downhole pump to produce fluids from the well.
Advantageously, as noted in the specification of U.S. Pat. No. 5,573,063 (ref col. 6, lines 63-65 thereof), the belt and pulley speed-reduction system of U.S. Pat. No. 5,573,063 provides damping, via the belts, to avoid transitional high stresses being transmitted from the rod string to the pulleys and driving motor, and vice versa, which according to the teaching of such patent provides a clear advantage over direct gearing system being used as the uphole speed-reduction mechanism. Specifically, the above system of U.S. Pat. No. 5,573,063 using belts and pulleys to reduce speed at surface is particularly used to reduce high cyclic stresses between the rod string and the uphole motor and damp such high transitional stresses which would otherwise be directly transmitted via a gearbox to the rod string, and vice versa (ref. col. 6, lines 63-65).
Undesirably, however, the belt and pulley speed-reduction system of U.S. Pat. No. 5,573,063 poses a corresponding problem with regard to wear of the associated semi-elastic flexible belts due to imposed transitional tensional stresses imparted thereon during operation. Accordingly, the system of U.S. Pat. No. 5,573,063 is often prone to failure due to such stresses being imposed on the belts and pulleys, resulting in the need to frequently service such systems and frequently stop operation of such system to replace belts.
Accordingly, a need still exists for a pumping system that can achieve the advantages of reduced rotational speed of the rod string yet still achieve sufficient downhole rotational speed of the downhole pump while further damping and/or reducing transmission of transitory high stresses between the rod string and the uphole driving unit, yet further be sufficiently robust to and avoid the need to frequently service such uphole drive systems.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.